1. Field of the Invention
The present invention relates to a light scanning unit, and more particularly, to a light scanning unit suitable for an exposure device for an electrophotographic apparatus, such as a color printer having a plurality of exposed objects.
2. Description of the Related Art
Electrophotographic exposure devices are generally categorized as devices using either a laser diode or devices using a light emitting diode (LED). The electrophotographic exposure device (EPE device)that uses an LED perform an exposure process by mapping one LED per one dot of a pixel of an image to be recorded in an exposed object. In general, the EPE device using the LED uses a light source called a “LED head”, in which a plurality of LEDs are arranged.
The light source is configured in such a manner so as to arrange an LED chip in which a plurality of LEDs are formed on a substrate and to form a plurality of LED arrays. Japanese Patent Publication No. Hei 10-035011 (published on Feb. 10, 1998) entitled “Light Emitting Diode Array and Fabrication thereof”, the entire contents of which are incorporated herein by reference, discloses this type of configuration. The disclosed apparatus condenses light onto an image-formed surface by providing an optical system between the LED and the exposed surface of the exposed object.
The EPE device that uses the laser diode scans a laser beam in a main scanning direction on an exposed surface of an exposed object using a light scanning unit.
The EPE device using the laser diode uses an F-θ lens to maintain the same scanning speed and same beam shape on an exposed surface. Japanese Patent Publication No. Hei 09-096769 (published on Apr. 8, 1997) entitled “Method and Device for adjusting Optical Axis of Optical Scanner and Optical Scanner”, the entire contents of which are incorporated herein by reference, discloses this type of configuration.
In the disclosed apparatus of Japanese Patent Publication No. Hei 09-09679, the laser beam emitted from a laser diode is diffused. The laser beam is then collimated by a collimator lens. The shape of the collimated laser beam is restricted to the shape of a slit, and the laser beam is focused by a cylinder lens in a subscanning direction on the reflective surface of a polygonal rotating mirror, which is a light scanning unit. Subsequently, light scanned in the main scanning direction by the polygonal rotating mirror is focused by an F-θ lens (or lens group) on the exposed surface of the exposed object, and is scanned at a uniform speed.
The electrophotographic exposure device using the LED can be made smaller. However, an EPE device using the LED made smaller has several problems. First, a plurality of LED chips should be precisely arranged on a substrate; second, the required circuitry is complex; third, due to the characteristics of the optical system, the distance from each LED chip to the exposed surface of the exposed object should be maintained precisely; and fourth, the quantity of light emitted between different LEDs is non-uniform, and should be corrected.
In addition, the optical system for scanning a laser diode as a light source which uses a polygonal rotating mirror to scan the light source has a problem as well. Maintaining or knowing the distance with respect to an exposed surface is comparatively low, since the spot of light quantity on an exposed surface is small, the circuit is simple, and the depth of focus is deep.
Meanwhile, color electrophotographic printers have been recently developed. These color electrophotographic printers need to form an image four times, as compared to a conventional black/white printer (which only has to form an image only once). Color printers need to form an image four times because color images are typically produced with four colors, such as cyan, magenta, yellow, and black, into one image.
Electrophotographic color printers are categorized into two types, a single pass electrophotographic color printer or a multi-pass electrophotographic color printer. The single pass electrophotographic color printer mounts one exposure device in one drum, performs a development process using a four-color developer, superimposes an image on an intermediate transfer body, and transfers the superimposed image onto a sheet of paper.
The multi-pass electrophotographic color printer mounts four developers and four exposure devices, and four photosensitive bodies. Therefore, the mechanism appears as four conventional black/white printers superimposed on one another.
In the single pass electrophotographic color printer, the output speed is reduced to one-quarter of that of a multi-pass electrophotographic color printer, requiring the image to be superimposed four times. As a result, the single pass electrophotographic color printer is low speed and the mechanism for moving a developer is complex. The single pass electrophotographic color printer can, however, configure the photosensitive drum and exposure device as a single body.
The printing speed of the multi-pass electrophotographic color printer is faster as compared to the single pass electrophotographic color printer. However, the multi-pass electrophotographic color printer requires four exposure devices and four photosensitive bodies and thus, its structure becomes complex.
The present invention relates to an exposure device for the multi-pass electrophotographic color printer.
Conventional multi-pass electrophotographic color printers typically incorporate the same number of laser diodes as the number of exposed objects. Since there are four exposed objects in the conventional multi-pass electrophotographic color printer, there are four laser diodes. Similarly, the multi-pass electrophotographic color printer also requires a polygonal rotating mirror or an F-θ lens having the same number of mirrors or lens as the number of exposed objects. Again, since there are four exposed objects, the conventional multi-pass electrophotographic color printer has either four lens or four polygonal mirrors. As a result, the size of an apparatus becomes larger, and consequentially, manufacturing costs are increased.